This invention relates to a heating apparatus, particularly a food cooker and/or rethermalizing apparatus.
Food Service food preparation in recent years has involved convenience foods or convenience packaged foods for easier preparation or cooking. Typically, this involves one of two known techniques using flexible film packages, namely: 1) so-called "cook-chill," and 2) so called "sous vide". These two are known to be excellent systems for supplying high quality, nutritious, previously prepared food, ready for reheating to provide meals. The technique known as cook-chill involves soups, sauces, and other products of pumpable consistency. The product is cooked at a facility separate from restaurants or the like, where it is pumped into strong plastic casings, closed by clipping or heat sealing, then chilled and kept under refrigeration, optionally frozen. Non-traditional cook-chill products such as pasta, rice, and vegetables are also becoming popular.
In contrast to this cook-chill technique described above, the sous vide technique involves packing of raw or partially cooked products into plastic pouches under vacuum, then cooking, and finally chilling either to frozen or nonfrozen condition.
Sous Vide technique is typically used for a variety of meat or fish entrees. These refrigerated, vacuum packaged foods are then reheated in selected groupings at the restaurant or other location, in response to customer orders. Experience has demonstrated that such carefully prepared food items can be of excellent quality, satisfactory to even the most discerning palates.
When properly treated, these previously prepared foods enable serving of high quality food without having the expense and complications of a talented chef on staff at restaurants and/or institutions, or without requiring extended cooking time if done at home. Preparation can be rapid, relatively inexpensive, and far less complex than in a typical operation.
The excellent quality of this food preparation technique is too often lost in the reheating process. The difficulty has been found to occur largely as a result of inaccurate temperature control and/or variable heat transfer rates during the reheating process. In fact, proper heating has been found to be critical to high quality results. Heating in boiling water tends to destroy food quality. Placing the packages in hot water below the boiling temperature for a selected time may or may not result in satisfactory food. The resulting temperature of the food product may be too high or too low. What can occur is that a meat serving, for example, is overheated and tough while another portion is not fully heated. Efforts have been made to circulate the reheating water medium by using circulation pumps as in Vilgrain et al U.S. Pat. No. 5,097,759; but this is not considered an adequate solution for consistent food quality. The bath water does not flow evenly over all surfaces, but rather takes the path of least resistance. Further, the bath water tends to stratify into thermal layers of different temperatures. It has been determined that the bath tends to heat unevenly. Even if the heat is spread over the surface of the vessel, it will separate into laterally adjacent zones, each having a width of about six inches or so. These all tend to prevent controlled temperature cooking or reheating in a hot water bath. Controlled proper heating or rethermalizing of the servings is further complicated by the fact that several different packages of different food are typically put into the bath during a particular time period, and these are inserted at different times in response to successive customer orders received. Hence, even though complexities due to meals not being totally prepared by the restaurant kitchen are bypassed, still the cooking or rethermalization of many individual meals and various components of those meals can become chaotic and difficult to properly perform.
Portions of food products in flexible film packs have only, until now, been cooked successfully in Sous Vide or Cook Chill preparation facilities wherein the equipment is large, special purposed, and monitored by computers including special temperature probes inserted into food portions in the processing chamber.
In general, standard restaurant or home reheating/cooking equipment is unsuited to cooking and poor at reheating food products in flexible film packages. The standard equipment is characterized by poor control of operating temperatures. Cooking is accomplished using a cooking fluid of water, air or oil. Temperature differences in an oven typically amount to 20.degree. F. and can significantly affect quality unless a chef is there to interrupt or modify the cooking process appropriately. Even so, air transfers heat so much slower than water and has very little stored energy, so the oven is somewhat forgiving when cooking unpackaged food, but is unsuited to cooking or reheating flexible film packaged food.
Fryers use oil or shortening as the temperature transfer media and have very sophisticated control systems to hold temperatures within a few degrees. Even so, they require either a chef of great experience to monitor the cooking or the even more sophisticated frying computers as in Polster U.S. Pat. No. 4,362,094 to achieve consistent quality cooking. The operating temperatures of fryers is unsuited to cooking or reheating flexible film packaged food.
Pots or pans of hot or boiling water are good for cooking eggs and vegetables. They are unsuited for cooking or reheating flexible film packaged food.
Typical Bain Marie's have a heater in the bottom of a large bath of water. Often a temperature control is used to control the temperature of the bath. If a load of food product is introduced into one portion of the bath, the remaining portion of the bath will either go to a much higher temperature (often boiling) or the area where the food has been introduced will drop in temperature depending on where the temperature control sensor is located. In either case all or some of the food will be heated at the wrong temperature and in one case the remaining area of the bath will be at the wrong temperature for the introduction of more food products.
In restaurant kitchens, another known severe problem that significantly affects cooking is water level control for heating bath vessels. Water level is typically established manually by adding cold or hot water to the heated vessel to replace water lost by evaporation and by water transfer with the packaged food items removed. Adding this water to a heated vessel, usually in a significant quantity by the time the bath decline is noticed, typically causes a significant temperature change, usually a decline, in the bath. Even if heated water is added, its temperature will most likely be many degrees different from the bath temperature. This alters the cooking/reheating process and thus alters the time required to cook or reheat the food items in the bath.
When cooking pasta in boiling water, the action of the boiling keeps the pasta from sticking together. Most modem pasta cooking systems have a perforated basket that drops into the boiling water and contains the pasta to be cooked. The boiling action does not occur within the perforated basket and the operator is obligated to stir the pasta while cooking to keep it from sticking together. Therefore modem pasta cooking systems actually cook pasta in hot water which does not have enough circulating action to properly cook it.
Another more subtle obstacle to proper cooking/reheating in a water bath has been discovered to be the standard proportioning temperature control technique which compensates for lag time and the thermodynamics of the water bath itself. The problem is that such techniques are not responsive to highly variable loads. Therefore in order to avoid the disasterous overshoot of temperature of an on-off control system, the recovery time of a proportioning temperature controlled water bath varies dramatically with load which changes the time to cook significantly. This problem is unobvious as the water bath seems to perform by properly coming to temperature without overshoot and the variations in cooking are mysterious.
When dealing with hot water baths, even 5.degree. F. significantly changes the texture of many foods to be cooked. Temperatures should be held within about 2.degree. F. of the optimum temperature. Circulation of the fluid helps to prevent temperature layer stratification, but circulation alone without exact bath temperature control is not enough. Further, the presence of circulation does not necessarily assure even flow over all surfaces of the food items. This flow over all surfaces of the food items is important for proper cooking or reheating. The mere presence of the food items disturbs the circulation pattern of any cooking vessel, with the fluid taking the path of least resistance and not the paths between all the food items, especially if the paths are narrow. Fluid dynamics in the cooking vessel present problems.
It has been found by the inventor herein to be important, for proper cooking or rethermalization, that the temperature be retained in a closely controlled range, for there to be totally efficient heat exchange with the surfaces of each of the several packages or items of food being cooked or reheated, for each item to be heated in accordance with the type of food in the package, i.e., for each type of entree, sauce, pasta, and vegetable etc. to be heated for an exact predetermined amount of time and at a temperature which is optimum for that entree or sauce. Achieving these results with present cooking or rethermalization equipment does not dependably occur. As noted, if too high a temperature is used, the food becomes overcooked and the texture is adversely affected. If too low a temperature is used, the food is not properly cooked. If the temperature changes, the food chemistry changes.
It has also been found that vegetables can be cooked in the water bath, using the above mentioned principles, loose or in a perforated flexible film package, or reheated after initial cooking and vacuum packaging.
Another discovery was that the bubbles used to vertically permeate the water bath between food packages kept pasta from sticking together even though the bath was not boiling. The bubbles also facilitated the removal of the starch that is cooked out of the raw pasta. The starch and a small amount of water was then found to be removed from the bath by the bubble driven circulation system. The water level control system made up the water lost removing the starch.